This invention relates to electrophoretic display devices in general and, more particularly, to an electrophoretic display apparatus which employs means for selectively erasing and rewriting individual characters on the display panel thereof.
Electrophoretic displays (EPIDS) are now well known. A variety of display types and features are taught in several patents issued in the names of the inventors herein, Frank J. DiSanto and Denis A. Krusos and assigned to the assignee herein, Copytele, Inc. of Huntington Station, N.Y. For example, U.S. Pat. Nos. 4,655,897 and 4,732,830, each entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS describe the basic operation and construction of an electrophoretic display. U.S. Pat. No. 4,742,345, entitled ELECTROPHORETIC DISPLAY PANELS AND METHODS THEREFOR, describes a display having improved alignment and contrast. Many other patents regarding such displays are also assigned to Copytele, Inc.
The display panels shown in the above-mentioned patents operate upon the same basic principle, viz., if a suspension of electrically charged pigment particles in a dielectric fluid is subjected to an applied electrostatic field, the pigment particles will migrate through the fluid in response to the electrostatic field. Given a substantially homogeneous suspension of particles having a pigment color different from that of the dielectric fluid, if the applied electrostatic field is localized it will cause a visually observable localized pigment particle migration. The localized pigment particle migration results either in a localized area of concentration or rarefaction of particles depending upon the polarity and direction of the electrostatic field and the charge on the pigment particles.
The electrophoretic display apparatus taught in the foregoing U.S. Patents are "triode-type" displays having a plurality of parallel cathode lines and a plurality of transverse grid electrode lines insulated from the cathode lines. The cathode and grid lines are referred to as row and columns and the terms may be interchanged. The grid cathode structure forms an X-Y matrix enabling one to address tile display at each X-Y intersection and thereby cause pigment particles to migrate relative to the cathode. Given a specific particulate suspension, the sign of the electrostatic charge which will attract and repel the pigment particles will be known. The cathode voltage, the anode voltage, and the grid element voltage can then be ascertained such that when a particular voltage is applied to the cathode and another voltage is applied to the grid, the area proximate their intersection will assume a net charge sufficient to attract or repel pigment particles in suspension in the dielectric fluid.
Since numerous cathode and grid lines are employed, there are numerous discrete intersection points which can be controlled by varying the voltage on the cathode and grid elements to cause localized visible regions of pigment concentration and rarefaction. Essentially then, the operating voltages on both cathode and grid must be able to assume at least two states corresponding to a logical one and a logical zero. Logical one for the cathode may either correspond to attraction or repulsion of pigment. Typically, the cathode and grid voltages are selected such that only when both are a logical one at a particular intersection point, will a sufficient electrostatic field be present at the intersection relative to the anode to cause the writing of a visual bit of information on the display through migration of pigment particles. The bit may be erased, e.g., upon a reversal of polarity and a logical zero-zero state occurring at the intersection coordinated with an erase voltage gradient between anode and cathode. In this manner, digitized data can be displayed on the electrophoretic display.
Accordingly, in order to erase the image in an electrophoretic display of the type in which negatively charged, light colored pigment particles are suspended in a dark-colored suspension medium, the anode is biased negatively relative to the cathode so that the particles are attracted to the cathode. In the aforementioned type of electrophoretic display device, the anode is typically a thin, unitary layer of ITO to which a first voltage is applied in the write mode and a different voltage is applied in an erase mode. All lines of the displayed image are erased simultaneously upon application of the erase voltage anode, and all lines of the display must be rewritten to form the next image frame. The next frame may often have character lines for image portions which are the same as the previous frame, which results in the redundancy of rewriting numerous identical lines from frame to frame.
There are also anode electrode structures which comprise conductor strips instead of a solid thin layer of ITO. One such anode structure is described in U.S. Pat. No. 5,053,763, issued to Frank J. DiSanto and Denis A. Krusos, entitled DUAL ANODE FLAT PANEL ELECTROPHORETIC DISPLAY, which is also owned by the assignee of the present application. In an electrophoretic display panel which is used to display text, characters are formed utilizing a predetermined number of such anode conductor strips in a group, the predetermined number of anode conductor strips being referred to as a character line and each of the predetermined number of anode conductor strips in the character line being referred to as an anode line segment. For example, in a typical such electrophoretic display panel, a character line is comprised of 26 anode line segments, each of which is approximately 0.125" wide and each of which is spaced approximately 0.001" from adjacent segments. A "selective" erase operation of a multiplicity of character lines, each of which comprises a multiplicity of anode line segments, is performed by applying a negative voltage to the anode line segments of the selected character lines.
While the segmented anode structure described above permits a frame to be written much faster than previous display devices, the selective erase operation it performs is not equally effective in all situations. For example, where only a few characters of a single line are to be erased or rewritten and a negative voltage is applied to the anode line segments of the selected character line, a part of adjacent character lines on either side of the selected character line are also erased. As such, the display can temporarily appear illegible or hard to read. Moreover, the erasure of an entire line is time consuming and inefficient when only a few characters or less of a character line are to be erased or rewritten.
In U.S. Pat. No. 5,174,882, entitled ELECTRODE STRUCTURE FOR AN ELECTROPHORETIC DISPLAY APPARATUS and issued on Dec. 29, 1992 to Frank J. DiSanto and Denis A. Krusos, there is illustrated another display device which includes a segmented anode structure. This patent prevents the partial erasure of adjacent character lines by configuring the anode as a plurality of alternating conductors. When selected for erasure of a corresponding character line, one of the conductors is biased with an erase potential of a given polarity and adjacent conductors are biased with a potential of an opposite polarity. The latter conductors prevent the partial erasure of the adjacent character lines. As in the case of the '763 device, however, the operation of this device also requires the erasure and rewriting of all characters of a selected line, which erasure is time consuming and inefficient when only a few characters need be erased or rewritten.
Accordingly, it is an object of the present invention to provide a method and an electrophoretic display which overcomes the aforementioned disadvantages of the prior art devices. In particular, the object of the invention is to provide an electrophoretic display in which individual characters on a line of the display can be selectively erased and rewritten without materially disturbing the appearance of other image characters of that line which remain the same from one frame to the next.